fix(web): close the gap between the composer and its bottom toolbar

[ScottN-PV]

What Changed

The bottom toolbar's backdrop mask now starts one CSS pixel higher, under the composer. Both lower mask stops use a new --spacing-composer-seam token, defined as calc(var(--spacing) * 4 - 1px). The dark background gradient keeps its position. The diff also adds one-line doc comments to the three ComposerSurface wrappers.

Why

Fixes #12937.

With an attached banner and the composer collapsed, scrolling conversation text showed through a thin gap between the input and the bottom toolbar. The mask ended exactly on the seam. The upper banner seam already uses the same one-pixel overlap.

UI Changes

Before: a thin sliver of sharp conversation text is visible at the lower join. After: the join stays continuous in the tested states.

Before:

After:

Both were captured at the same scroll position and display scale, with slightly different crop bounds. Recordings did not reliably preserve the one-pixel sliver, so no video is attached.

Validation

• Reproduced the gap on Windows at 150% display scaling in dark mode, in an isolated development server inside T3 Code's Browser panel. The fixture is a synthetic long conversation that shows the "Resume with less context" banner. No Claude agent ran.
• Manual checks at 100%, 125%, and 150%, in light and dark mode: attached banner with the composer collapsed while scrolling, composer expanded, and unfocused scrolling with the banner dismissed. The join stayed continuous.
• Restoring the original code brought the sliver back. Reapplying the patch removed it.
• These manual checks and screenshots come from the first version, tested from base 371b52d9dad76876f84609a4cd0f80eaa757b69c. The branch was later merged with main at 6989856a and moved to the token. No manual visual pass was repeated after that.
• After the merge, compiling the real apps/web/src/index.css with @tailwindcss/node 4.3.3 emits the token, and both mask positions reference it.
• Targeted lint, formatting, and the web typecheck pass.

Not tested: a packaged desktop build, standalone browsers, macOS, Linux, and mobile. Open PR #8034 changes how timeline content fades beneath the composer. This change is limited to the lower toolbar mask.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Model: gpt-6-astra (reasoning medium). Harness: OpenAI Codex in the T3 Code desktop app. The first version ran on Codex CLI 0.155.1 through its app-server integration. Later sessions did not expose the integration or version. Codex did the investigation, fixture, implementation, merge, automated checks, and doc comments.

Model: claude-fable-5-1 (effort high). Harness: Claude Code CLI 2.1.283 and 2.1.284, invoked by Codex through PowerShell. Reviewed the doc comments and drafted this description.

Human involvement: reproduced the defect, performed the manual checks, supplied the screenshots, scrolled the test conversation for the recordings, and reviewed the original wording. Authorized the merge with main. Requested the doc comments and their review, and reviewed and approved the first of the two doc-comment updates. No manual testing or code review of the merge is claimed.

Summary by CodeRabbit

• Style
  • Adjusted the chat context strip’s visual mask boundary to align with the composer seam, placing it one pixel below four times the base spacing.
  • The change refines the outline transition while preserving the context strip’s existing behavior.

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Reviewing files that changed from the base of the PR and between d5df365 and 8fa3e6c.

📒 Files selected for processing (1)

• apps/web/src/components/chat/ComposerSurface.tsx

🚧 Files skipped from review as they are similar to previous changes (1)

• apps/web/src/components/chat/ComposerSurface.tsx

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📝 Walkthrough

Walkthrough

The ContextStrip mask gradient now uses the --spacing-composer-seam token for both transition lengths. The token is calculated as four times the base spacing minus one pixel. Comments describe the roles of Host, Main, and ContextStrip.

Changes

Composer mask adjustment

Layer / File(s)	Summary
Adjust ContextStrip mask boundary apps/web/src/index.css, apps/web/src/components/chat/ComposerSurface.tsx	The new --spacing-composer-seam token is set to calc(var(--spacing) * 4 - 1px). The ContextStrip mask gradient uses this token for both transition lengths. Comments describe the roles of Host, Main, and ContextStrip.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Bug fix · Severity of issue fixed: Low

Merge Risk: ⚪ Minimal · up to 8fa3e

The one-pixel mask adjustment has no identified merge-blocking risk. The reported visual checks were not repeated for this refresh.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	For [#12937], ContextStrip now uses --spacing-composer-seam for both lower mask transitions. --spacing-composer-seam is calc(var(--spacing) * 4 - 1px), which provides the requested one-pixel o…
Out of Scope Changes check	✅ Passed	The pull request changes only ComposerSurface.tsx and index.css. The component change applies the lower mask overlap for [#12937]. The CSS token supports that same change. The added comments docum…
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 1 files.
Title check	✅ Passed	The title clearly identifies the web fix and the affected composer-to-toolbar gap. It accurately summarizes the main change.
Description check	✅ Passed	The description includes the required What Changed, Why, UI Changes, and Checklist sections. It explains the defect, implementation, screenshots, validation, and testing limits. The missing video is j…

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Inline comments:
In `@apps/web/src/components/chat/ComposerSurface.tsx`:
- Line 88: The mask utilities in ComposerSurface reference
--spacing-composer-seam, which may not be emitted from an inline theme. Declare
this token in a static theme and remove its inline-theme declaration so both
mask positions resolve.

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Reviewing files that changed from the base of the PR and between b9da964 and 7ab6227.

📒 Files selected for processing (2)

• apps/web/src/components/chat/ComposerSurface.tsx
• apps/web/src/index.css

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